Cable networks have, in recent years, moved beyond merely broadcasting television signals over a co-ax cable to subscribers in their homes. Subscribers of a cable network nowadays have a modem allowing the transmission of digital signals upstream toward the headend of the network. Among many services afforded by cable modems are: an Internet service, a home shopping service using a television catalogue, and a voice-over-IP phone service.
In bidirectional cable networks, the upstream and the downstream signals occupy separate frequency bands called upstream and downstream spectral bands. In the United States, the upstream spectral band typically spans from 5 MHz to 42 MHz, while the downstream spectral band typically spans from 50 MHz to 860 MHz. Downstream information channel signals co-propagate in the downstream spectral band, and upstream signals co-propagate in the upstream spectral band. The frequency separation of the upstream and the downstream signals allows bidirectional amplification of these signals propagating in a common cable in opposite directions.
To provide upstream communication capability to a multitude of subscribers, the upstream frequency channels are used in a so called time-division multiplexing (TDM) mode. Each cable modem is assigned a time slot, within which it is allowed to transmit information. The time slots are assigned dynamically by a cable modem termination system (CMTS) disposed at the headend. The time slot information is communicated by CMTS to individual cable modems via an allocated downstream channel. Subscribers access available network resources by using a data communication bridge established between CMTS and individual cable modems. Subscribers send data from their digital devices into cable modems, which then relay the data to the CMTS. The CMTS, in turn, relays the information to appropriate services such as Internet servers, for example. Information destined to the subscriber digital device is provided from the Internet servers to the CMTS, which in turn relays the information to individual cable modems. The cable modems then relay the information to the digital devices used by the subscribers.
One popular communication standard for bidirectional data transport over a cable network is the Data Over Cable Service Interface Specification (DOCSIS). DOCSIS establishes rules of communication between CMTS and cable modems in a cable network. Three revisions currently exist for a North American DOCSIS standard, DOCSIS 1.x, 2.0, and 3.0. In addition to the 6-MHz wide North American based DOCSIS standard, there exists a European (Euro-DOCSIS) standard formatted for 8-MHz wide bandwidth channels.
As cable communication systems grow and become more complex, the task of proper system maintenance and troubleshooting becomes more difficult. The difficulty results from a random nature of signal bursts from individual cable modems. Although the cable modems are allocated time slots in which they are allowed to transmit, the actual transmission depends on network activity of individual subscribers. Furthermore, the upstream signal bursts from the cable modems have a very short duration and arrive intermittently from a multitude of locations in the cable network. Consequently, an upstream signal from a faulty location is interspersed with upstream signals from locations that are functioning normally. To be able to detect and eliminated faults in a cable network, it is important to identify faulty network locations.
While it is the headend where faulty signals' locations can be detected, it is the remote locations where the faults typically occur. A technician willing to fix a network problem must first analyze the symptoms of the problem at the headend, then determine a geographical location of the fault, then drive there and attempt to fix the problem, then drive back to the headend and make sure the problem is fixed. One can employ two technicians equipped with a mobile communications device allowing them to communicate with each other, one technician remaining at the headend, and the second technician moving around in the field. This solution is costly because it increases labor costs. Furthermore, it is often difficult for the technician located at the headend to verbally describe the signal degradation patterns he observes to the technician located in the field.
The need to test the upstream signal path from a node disposed remotely from a headend is recognized in the art. In U.S. Pat. No. 7,489,641 by Miller et al., upstream path test apparatus and method are described. In the method of Miller et al., test data packets are generated by a test device connected to a remote node. The test data packets have the destination address of the test device itself. Therefore, when the test data packets are transmitted to the headend, the headend automatically routes them back to the test device. The test packets are then received, demodulated, and analyzed by the test device for faults. Disadvantageously, the test apparatus of Miller et al. cannot distinguish whether the degradation has occurred in the upstream path or the downstream path of the network.
In US Patent Application Publication 20050047442, Volpe et al. describes a test system that is configured to receive all upstream/downstream channels and demodulate upstream packets. A database of MAC/SID addresses is built, which allows the test system to eventually determine where the packets came from. Once the database is built, the origin of faulty data packets can be determined. Disadvantageously, the test system of Volpe lacks a capability to troubleshoot a particular upstream signal problem in real time.
Test systems for upstream signal analysis are known in the art. By way of example, PathTrak™ and QAMTrack™ test systems, manufactured by JDSU Corporation located in San Jose, Calif., USA, allow for upstream signal demodulation, analysis, and MAC address filtering at the headend. The results of the analysis can be made available for remote clients through a web browser interface. To test a particular node or a cable modem, one can specify a MAC address of a cable modem under test. Provided that enough time is given to the PathTrak system, an upstream signal burst from the cable modem under test can be captured at the headend and subsequently analyzed for faults. Unfortunately, due to random nature of upstream signal bursts, and due to long time required for packets demodulating and MAC filtering, analysis of faults at a particular node cannot always be performed in an efficient way. Furthermore, even when the data packet is correctly identified, the PathTrak system does not have an access to pre-equalization coefficients used by the cable modem under test to generate the upstream burst. Without the pre-equalization coefficients, the test system cannot determine correctly the upstream path signal distortions and the in-band spectral response.
The prior art is lacking a test method and a system allowing a technician to analyze and troubleshoot upstream path problems by performing a real-time analysis of test data packets generated by a specific device in the field and received at the headend of a cable network. The present invention provides such a device and a method.